The invention herein relates to the field of imaging synthetic aperture radars. More particularly, the invention relates to a linear-FM, synthetic aperture radar (SAR) which implements overlapped-subaperture real-time processing to correct for motion through resolution cells.
As with many applications of imaging devices, the finer the resolution the more useful the imaging devices become; however, the spatial resolution achievable by an antenna, without any coherent processing, is inversely proportional to the antenna's aperture. Thus, for fine resolution, a large antenna is required. To overcome the inability to place a large antenna on an imaging system in motion such as an aircraft or satellite, SAR samples a large aperture by positioning a small antenna at well defined locations along the large aperture. These samples are then coherently combined in the radar's signal processor to generate, synthetically, the resolution appropriate for the aperture traveled by the small antenna. In order to coherently combine the samples, the relative position of the antenna must be known to within a fraction of a wavelength. If the synthetic aperture to be sampled by the radar is a straight line, components of motion away from the line degrade the coherent combination of the samples of the synthetic aperture. The angular relationship between the line-of-sight vector to any point in the scene to be imaged and the undesired motion vector appears as phase modulation of the radar echo when viewed at the radar receiver. Spatially varying phase errors, also dependent on the motion of the radar, occur because the effect is not constant for all locations in the scene.
To achieve finer resolution image processing, the signal processing burden must increase at least as rapidly as the inverse of the resolution squared, assuming that the image area is constant and the X and Y dimensional resolution change at the same rate. Thus, it appears that the requirements for both a real-time, imaging SAR, and fine resolution are in conflict. For this reason, radar systems which efficiently utilize computing devices are particularly important to advancing the state-of-the art in real-time SAR.
Synthetic aperture radars are known in the art. For example, much background is described in "Developments in Radar Imaging", IEEE Transactions On Aerospace and Electronic Systems, Volume AES-20, Number 4, July 1984.
U.S. Pat. No. 4,771,287 to Mims describes a method of correcting for focus errors and higher order errors that occur in radar processing. The system includes first and second stages for processing radar signals over a plurality of time samples. The first stage derives the signals at each time sample and forms a set of complex signals of point-like scatterers to form a time history. Focusing errors are corrected by generating a set of correcting signals. The method, however, compensates for phase errors common to range and azimuth locations within the imaged area, but cannot compensate for phase errors that vary as a function of location.
U.S. Pat. No. 4,758,838 to Maeda et al. describes a method of reconstructing original images from synthetic aperture radar image data. In order to speed up the reconstruction of the original image the process for generating a point image pattern and the Fast Fourier Transformation process are carried out once for every certain number of lines. Positional displacement is corrected by multiplying a phase rotation factor by the product of the Fourier image pattern and one line of the image data. The method uses range compression before azimuth compression. However, the method does not use two stages of azimuth compression which implements a subaperture approach.
U.S. Pat. No. 4,034,370 to Mims describes a method using two stage azimuth processing with range migration compensation being processed after range compression. The method does not combine initial stage azimuth filtering with linear-FM transmitter pulse processing to implement a range-migration correction with a phase rotation.
The prior art processing schemes require inefficient interpolation to compensate for range migration. Moreover, prior art radar architectures suffer from unduly large computational burden and memory requirements of the signal processor by not correcting the received radar phase for undesirable, motion-induced, errors using phase rotations in the local oscillator and sampling rate changes in the A/D converter.
Prior art radar architectures calculate in general purpose computers the time-varying coefficient vectors which are needed in the signal processor and do not synthesize these vectors with digital hardware. Note that range and coarse-resolution azimuth positions are known before the final processing stage, so that some spatially dependent phase errors that limit other techniques such as polar format processing are eliminated.
It is thus an object of the present invention to account for spatially varying effects of motion induced phase errors to achieve high image quality over large scene areas. This object is achieved in the present invention by, inter alia, correcting the radar phase for undesirable, motion-induced, errors using phase rotations in the transmitter, receiver, and signal processor and sampling rate changes in the A/D converter.
It is another object of the present invention to provide a radar and method for imaging that can be efficiently realized in digital hardware.
It is yet another object of the present invention to provide an apparatus and method for radar imaging which implements the method with signal processing operations.
It is yet another object of the present invention to minimize the computational burden and memory requirements of signal processing equipment.
It is yet another object of the present invention to organize processing requirements so that slow time varying coefficient vectors may be produced in general purpose computers and rapidly time varying coefficient vectors may be synthesized in hardware synthesizers.
It is thus an object of the invention to improve image quality at fine resolution of synthetic aperture radar which is degraded because the relative range of near and far images of a scene changes with motion of the radar. The radar herein is designed to change the radar center frequency, f.sub.o, and either the signal sampling rate or the chirp rate based on actual radar motion using a digitally-controlled waveform synthesizer. A further advantage is that this improved image quality can be accomplished in a real-time system without adding to the burden of the radar's signal processor.
It is a further object of the invention to obtain fine resolution with minimal memory requirements. Because of the filtering operation at coarse resolution processing, only A&lt;M coarse-resolution azimuth bins need to be saved, thus achieving the reduction in real-time memory. Moreover, some of the signal processing operations are realized by varying the radar center frequency and either the chirp rate or the A/D sample rate. This process reduces memory requirements.
It is yet another object of the invention to achieve a synthetic aperture radar compensating for motion through range cells. This object is achieved when the spectrum of the signal in each subaperture is shifted by an amount that is proportional to the azimuth bin number of that column.
It is yet another object of the invention to allow for real-time processing concurrent with data collection. Certain functions, namely the coarse-resolution azimuth and the range processing, operate on one subaperture at a time wherein the data are reduced.
It is yet another object of the invention to remove the geometric distortion caused by range-doppler interaction. The feature of the invention which removes the geometric distortion is the use of the chirp-z transform as a last step in the processing. Moreover, a further advantage of this feature is that the invention produces an image with constant azimuth-sample spacing.
These and other objects are achieved with an imaging radar including a linear-FM type doppler radar for producing a received signal; a coarse-azimuth process for producing a plurality of subaperture arrays from the received signal, each subaperture array including a plurality of coarse-azimuth bins; a range process for producing a plurality of range bin vectors, each range bin vector having a plurality of range bins and being produced from a corresponding coarse-azimuth bin of the plurality of coarse-azimuth bins of each of the plurality of subaperture arrays according to the coarse-azimuth bin; and a fine-azimuth process for producing a radar image including a plurality of image lines, each image line being produced from a corresponding range bin of the plurality of range bins of each range bin vector of the plurality of range bin vectors.
These and other objects are achieved with a method of radar imaging including the steps of producing a received signal from a doppler radar having a linear-FM waveform; producing a plurality of subaperture arrays from the received signal, each subaperture array including a plurality of coarse-azimuth bins; producing a plurality of range bin vectors, each range bin vector having a plurality of range bins and being produced from a corresponding coarse-azimuth bin of the plurality of coarse-azimuth bins of each of the plurality of subaperture arrays according to the coarse-azimuth bin; and producing a radar image including a plurality of image lines, each image line being produced from a corresponding range bin of the plurality of range bins of each range bin vector of the plurality of range bin vectors.